spi: add basic support for SPI offloading

spi: add basic support for SPI offloading

Add the basic infrastructure to support SPI offload providers and
consumers.

SPI offloading is a feature that allows the SPI controller to perform
transfers without any CPU intervention. This is useful, e.g. for
high-speed data acquisition.

SPI controllers with offload support need to implement the get_offload
and put_offload callbacks and can use the devm_spi_offload_alloc() to
allocate offload instances.

SPI peripheral drivers will call devm_spi_offload_get() to get a
reference to the matching offload instance. This offload instance can
then be attached to a SPI message to request offloading that message.

It is expected that SPI controllers with offload support will check for
the offload instance in the SPI message in the ctlr->optimize_message()
callback and handle it accordingly.

CONFIG_SPI_OFFLOAD is intended to be a select-only option. Both
consumer and provider drivers should `select SPI_OFFLOAD` in their
Kconfig to ensure that the SPI core is built with offload support.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-1-e48a489be48c@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>